I have a nice digital camera, but would like something better for an Alaskan cruise. Any suggestions - needs to be simple to use, but able to get wildlife pictures. Do you know if it's possible to rent a nice camera?

look for something with an optical zoom along with the digital zoom, with the optical zoom you can zoom in and out like on a regular 35mm camera, this would help with the wildlife shots.
As for rental, your best bet would be to call rental places like Taylor Rental